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10 - Day 22 - Rapid STP.md

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Day 22 - Rapid STP

Reviewed: No

Key Concepts

Different Versions of Spanning Tree Protocol

  1. 802.1D (Classic STP)
    • Original IEEE standard created to prevent Layer 2 loops.
    • Single STP instance for all VLANs (no load balancing).
    • Slow convergence (up to 50 seconds).
  2. PVST+ (Per-VLAN Spanning Tree Plus)
    • Cisco's enhancement of 802.1D.
    • Separate STP instance for each VLAN, allowing load balancing.
    • Slow convergence similar to classic STP.
  3. 802.1w (Rapid STP or RSTP)
    • Faster convergence than 802.1D, typically within seconds.
    • Single STP instance for all VLANs (no load balancing).
  4. Rapid PVST+
    • Cisco's enhancement of 802.1w.
    • Separate STP instance for each VLAN with rapid convergence.
    • Supports load balancing by blocking different ports in each VLAN.
  5. 802.1s (Multiple Spanning Tree Protocol - MSTP)
    • Groups VLANs into different instances to perform load balancing.
    • Superior to Cisco's Rapid PVST+ for large networks with many VLANs.
    • Cisco switches run the industry standard 802.1s without proprietary modifications.

Key Features of Rapid Spanning Tree Protocol (RSTP)

  • Purpose: Prevent Layer 2 loops by blocking specific ports.
  • Root Bridge Election: Same rules as in STP; lowest Bridge ID becomes the root bridge.
  • Root and Designated Ports: Similar to classic STP, with a focus on the lowest root cost.

RSTP Port Roles

  1. Root Port:
    • Closest to the root bridge.
    • One per non-root switch.
  2. Designated Port:
    • Sends the best BPDU on a segment.
    • One per collision domain (segment).
  3. Alternate Port:
    • A discarding port that receives a superior BPDU from another switch.
    • Functions as a backup to the root port, ready to move to forwarding state if needed.
  4. Backup Port:
    • A discarding port that receives a superior BPDU from another interface on the same switch.
    • Rarely encountered in modern networks due to the obsolescence of hubs.

RSTP Port States

  • Discarding: Combines the blocking, listening, and disabled states of classic STP.
  • Learning: Learns MAC addresses but does not forward frames.
  • Forwarding: Forwards frames and learns MAC addresses.

RSTP Features

  • Immediate Transition to Forwarding:
    • Handshake Mechanism: Allows rapid transition to the forwarding state.
    • Built-in Features: Includes functionalities of classic STP optional features like UplinkFast, BackboneFast, and PortFast.

RSTP Link Types

  1. Edge:
    • Connected to end hosts.
    • Functions like PortFast, moving directly to forwarding state.
  2. Point-to-Point:
    • Direct connection between two switches.
    • Operates in full-duplex mode.
  3. Shared:
    • Connection to a hub.
    • Operates in half-duplex mode (rarely used in modern networks).

BPDU Differences in RSTP

  • Protocol Version: 2 for RSTP (compared to 0 in classic STP).
  • BPDU Type: 2 for RSTP.
  • BPDU Flags: All 8 bits are used in RSTP for faster negotiation and convergence.
  • BPDU Generation: All switches in RSTP generate their own BPDUs, unlike classic STP where only the root bridge generates BPDUs.

Quiz Questions Overview

  1. IEEE 802.1D Optional Features in 802.1w:
    • Correct Answers: PortFast, UplinkFast, BackboneFast.
    • These features allow rapid port transitions to the forwarding state in RSTP.
  2. Configuring an 802.1w Edge Port:
    • Correct Command: spanning-tree portfast.
    • Configures a port connected to end hosts to move directly to the forwarding state.
  3. Identifying Root Bridge, Port Roles, and Link Types:
    • Root Bridge: Switch with the lowest priority.
    • Port Roles: Root ports, designated ports, alternate ports, and backup ports.
    • Link Types: Edge, point-to-point, and shared.

Summary

  • RSTP: An evolution of classic STP with faster convergence using negotiation rather than timers.
  • Port Roles and States: Simplified and optimized for rapid recovery and loop prevention.
  • Link Types: Introduces the concept of edge, point-to-point, and shared links.
  • Compatibility: RSTP is compatible with classic STP, with mixed environments possible but slower convergence on classic STP interfaces.

Commands Used in Day 22 (Rapid Spanning Tree Protocol)

  1. Set RSTP mode on a switch:

    spanning-tree mode rapid-pvst
    • Purpose: Configures the switch to use Rapid PVST+.

  2. Show Spanning Tree information:

    show spanning-tree
    • Purpose: Displays the current spanning tree information, including protocol (RSTP), port roles, and states.

  3. Configure an Edge Port (PortFast):

    interface [interface_id]
spanning-tree portfast
    • Purpose: Configures a port as an edge port to immediately move to the forwarding state when connected to an end device.

  4. Manually Configure Point-to-Point Link Type:

    interface [interface_id]
spanning-tree link-type point-to-point
    • Purpose: Explicitly configures an interface as a point-to-point link, typically between two switches.

  5. Manually Configure Shared Link Type:

    interface [interface_id]
spanning-tree link-type shared
    • Purpose: Configures an interface as a shared link type, usually connected to a hub (though rarely used in modern networks).

  6. Verify Spanning Tree Configuration:

    show spanning-tree interface [interface_id] detail
    • Purpose: Provides detailed information about the spanning tree status of a specific interface, including its role, state, and link type.